This invention relates to an improved process for the bromination of isoolefin polymers and to the product produced by such an improved process.
Isoolefin polymers have been known for many years and are commercially available as polymers of an isoolefin and a conjugated diolefin, especially of isobutylene and isoprene. While such polymers have a wide range of desired properties they are not readily covulcanizable with highly unsaturated polymers, such as polymers containing high proportions of one or more conjugated diolefin. In order to overcome such a problem, isoolefin-conjugated diolefin polymers, and especially isobutylene-isoprene polymers which are also known as butyl polymers, have been halogenated. The halogenation, especially chlorination or bromination, is achieved by treating the butyl polymer in solution in an inert organic solvent with controlled quantities of chlorine or bromine. The resulting halogenated butyl polymer has the inherent satisfactory properties of the butyl polymers while also being covulcanizable with the highly unsaturated polymers.
The halogenation process is well known to be rather inefficient. One reason for the inefficiency is that for every atom of chlorine or bromine that is incorporated into the polymer a molecule of hydrogen chloride or hydrogen bromide is formed. Another reason is that some of the hydrogen chloride or hydrogen bromide may add to the polymer forming a chemically undesirable group. A further reason is that the actual efficiency of utilization of the chlorine or bromine is quite low, generally being of the order of about 25 to about 40 percent by weight.
The preparation of butyl polymers is well known wherein a mixture of an isoolefin, preferably isobutylene, and a conjugated diolefin, preferably isoprene, in an inert diluent, preferably methyl chloride, is reacted at a temperature of from about xe2x88x9280xc2x0 C. to about xe2x88x92120xc2x0 C. in the presence of a Friedel-Crafts catalyst, preferably aluminum chloride. The butyl polymer so produced contains about 95 to about 99.5 mole percent of isobutylene and from about 0.5 to about 5 mole percent of isoprene. Such a polymer is dissolved in an organic solvent and reacted, at about 10xc2x0 C. to about 60xc2x0 C., with, preferably, chlorine or bromine for sufficient time to yield a polymer containing not more than 1 atom of combined chlorine per double bond in the polymer or not more than 3, and preferably not more than 1, atoms of combined bromine per double bond in the polymerxe2x80x94see for example U.S. Pat. Nos. 2,944,578 and 3,011,996. U.S. Pat. No. 3,018,275 describes a process for the halogenation of butyl polymers wherein an oxidizing agent, including a material selected from hydrogen peroxide, sodium peroxide, sodium chlorate or bromate and sodium hypochlorite, is present during the halogenation process to increase the utilization of the halogenating agent in the process.
It is an objective of this invention to provide an improved process for the bromination of isobutylene polymers by increasing the utilization of the bromine in the process.
It is another objective of this invention to provide an improved brominated butyl polymer by use of the improved process.
Accordingly, one aspect of the invention provides an improved process for the bromination of a C4-C6 isoolefin-C4-C6 conjugated diolefin polymer which comprises preparing a solution of said polymer in a solvent, adding to said solution bromine and reacting said bromine with said polymer at a temperature of from about 10xc2x0 C. to about 60xc2x0 C. and separating the brominated isoolefin-conjugated diolefin polymer, the amount of bromine being from about 0.3 to about 1.0 moles per mole of conjugated diolefin in said polymer, the improvement being that said solvent comprises an inert halogen-containing hydrocarbon, said halogen-containing hydrocarbon comprising a halogenated C2 to C6 paraffinic hydrocarbon or a halogenated aromatic hydrocarbon.
In one aspect, the present invention provides an improved process for the bromination of a C4-C6 isoolefin-C4-C6 conjugated diolefin polymer which comprises preparing a solution of said polymer in a solvent, adding to said solution bromine and reacting said bromine with said polymer at a temperature of from about 10xc2x0 to about 60xc2x0 C. and separating the brominated isoolefin-conjugated diolefin polymer, the amount of bromine being from about 0.3 to about 1.0 moles per mole of conjugated diolefin in said polymer, the improvement being that said solvent comprises an inert halogen-containing hydrocarbon selected from the group consisting of ethyl bromide, propyl chloride, n-butyl chloride and monochlorobenzene.
In another aspect, the solvent further contains up to 20 volume percent of water.
One aspect of the invention provides for the process wherein the solvent further contains up to 20 volume percent of an aqueous solution of an oxidizing agent. The oxidizing agent is soluble in water and is suitable to oxidize the hydrogen bromide to bromine in the process substantially without oxidizing the polymeric chain. In a particular aspect, the oxidizing agent is an oxygen containing oxidizing agent selected from the group comprising sodium hypochlorite, hydrogen peroxide, sodium peroxide, sodium chlorate, bromate, or other suitable oxidizing agents.
In yet another aspect, the solvent further contains up to 20 volume percent of an aqueous solution of sodium hypochlorite, the amount of sodium hypochlorite being equivalent to not more than about 200 percent of the moles of bromine added.
In a further aspect, this invention provides an improved brominated isoolefin-conjugated diolefin polymer produced by the aforesaid process wherein the brominated polymer contains from about 1 to about 4 weight percent of bromine based on the brominated polymer and not less than about 80 percent of the bound bromine atoms are present in a form suitable to participate in the vulcanization of said polymer and not less than about 70 percent of the 1,4-isoprene units in the original isoolefin-conjugated diolefin polymer are converted in the brominated polymer into a form suitable to participate in the vulcanization of said polymer.
Isoolefin-conjugated diolefin polymers are well known in the art as also is the process for the manufacture of such polymers. Isoolefins are selected from the C4 to C6 isoolefins with isobutylene being the preferred isoolefin. Conjugated diolefins are selected from the C4 to C6 conjugated diolefins with isoprene being the preferred conjugated diolefin. Such polymers comprise from about 95 to about 99.5 mole percent of the isoolefin, preferably from about 97 to about 99.5 mole percent of isobutylene, and from about 0.5 to about 5 mole percent of conjugated diolefin, preferably from about 0.5 to about 3 mole percent of isoprene. The polymer is prepared by the cationic polymerization of the isoolefin and conjugated diolefin, in an inert diluent which is preferably methyl chloride or ethyl chloride, at a temperature of from about xe2x88x9280xc2x0 C. to about xe2x88x92120xc2x0 C. in the presence of a Friedel-Crafts catalyst which is preferably aluminum chloride.
For the prior art bromination process, the polymer is dissolved in an inert hydrocarbon solvent such as pentane, hexane and heptane and the solution is fed to a halogenation reactor. The halogenation reactor is typically a vessel equipped with inlet and outlet lines and an agitator. Bromine is also fed to the halogenation reactor at a controlled rate in relation to the amount of polymer and the double bond content of the polymer. The material from the reactor is treated with an aqueous alkaline solution, such as sodium hydroxide, to neutralize the hydrogen bromide formed in the halogenation reaction and to react with residual bromine and then contacted with hot water and steam to remove the solvent and produce a slurry of brominated polymer in water which is then handled in a conventional manner to yield the essentially dry brominated polymer. Stabilizers for the brominated polymer may be added during the recovery process.
Such prior art processes exhibit poor utilization of the bromine in the halogenation process, due to the formation of one molecule of hydrogen bromide for each atom of bromine incorporated into the polymer, due to the addition of a small amount of the hydrogen bromide into the polymer and due to the need to use more bromine than is actually incorporated into the polymer.
We have now discovered that the halogenation process can be significantly improved by the use as the solvent for the polymer a solvent which comprises an inert halogen-containing hydrocarbon, more specifically a halogenated C2 to C6 paraffinic hydrocarbon or a halogenated aromatic hydrocarbon. Preferably, the inert halogen-containing hydrocarbon is selected from the group consisting of ethyl bromide, propyl chloride, n-butyl chloride and monochlorobenzene. The solvent may also contain up to about 20, preferably from about 3 to about 15, volume percent, based on the total solvent, of water. Further, the solvent may also contain up to about 20, preferably from about 3 to about 15, volume percent of an aqueous solution of an oxidizing agent such as an oxygen containing oxidizing agent selected from the group comprising sodium hypochlorite, hydrogen peroxide, sodium peroxide, sodium chlorate or bromate. The oxidizing agent is soluble in water and is suitable to oxidize the hydrogen bromide to bromine in the process substantially without oxidizing the polymeric chain. Weak oxidizing agents at low concentration, such as sodium hypochlorite, are preferred in order to prevent or minimize oxidation of the polymer. Preferably, the oxidizing agent will comprise an aqueous solution of sodium hypochlorite, the amount of sodium hypochlorite being equivalent to not more than about 200, preferably from about 100 to about 140, percent of the moles of bromine added to the process.
Using such a solvent for the polymer leads to an increase in the amount of bromine which is incorporated into the polymer to form chemical structures that participate in the vulcanization of said polymer. We have found that for a fixed reaction time in the process of the present invention in excess of about 85 mole percent of the bromine supplied to the process is incorporated into the polymer to form chemical structures that participate in the vulcanization of the polymer. In contrast, in the prior art only about 60 to 70 mole percent of the bromine is so incorporated. When the solvent includes water, the improvement in the utilization of bromine is believed to be due to the hydrogen bromide formed in the reaction being preferably soluble in the water and thus not so readily available to form hydrogen bromide addition structures. When the solvent includes aqueous sodium hypochlorite, the improvement in the utilization of bromine to form the desired chemical structures may be as high as about 190 mole percent of the bromine supplied to the processxe2x80x94this is believed to be due to the hydrogen bromide formed in the reaction being oxidized by the sodium hypochlorite to form bromine which may then further react with the polymer.
Without intending to limit the scope of the invention, the bromine utilization improvement is believed to be achieved by the increase of the dielectric constant of the reaction medium via the use of select halogen-containing hydrocarbons as solvent for the isoolefin-conjugated diolefin polymers instead of an inert hydrocarbon solvent. It is believed that the dielectric constant of the halogen-containing hydrocarbon could be used as a guide for the selection of proper reaction medium for the process of this invention. Table A lists the dielectric constant of some halogen-containing hydrocarbons and hexane. It is apparent from Table A that not all the halogen-containing hydrocarbons will be suitable for the process of this invention. For example, the dielectric constant of carbon tetrachloride is just slightly higher than that of hexane, therefore only marginal improvement can be expected. In contrast use of solvents with high dielectric constant, such as monochlorobenzene, n-butyl chloride, ethyl bromide, should lead to exceptional results. To achieve desired results, the dielectric constant will be greater than that of carbon tetrachloride.
In some of the halogenated solvents listed in Table A, solubility of the polymer may be limited. In such cases addition of some paraffinic hydrocarbon is recommended to aid dissolution of the polymer.
Such a discovery means that the amount of bromine supplied to the process may be reduced, that the amount of excess bromine leaving the halogenation reactor with the brominated polymer may be reduced which in turn reduces the quantity of aqueous alkaline solution required to react with it, that the amount of bromine chemically bound in the polymer may be more effectively used in the subsequent vulcanization process and that the unsaturation level in the polymer (that is the amount of bound conjugated diolefin) may be reduced because with the more complete utilization of the bromine more of the unsaturated double bonds in the polymer are effectively brominated than in the prior art processes.
The desired chemical structures in the brominated polymer include the exo allylic bromide structure, the endo allylic bromide structure and the re-arranged exo allylic bromide structure. In all of these cases the bromine atom is present attached to a carbon atom which is in an allylic configuration with a carbon-carbon double bond (i.e. Cxe2x95x90Cxe2x80x94CBr) wherein exo and endo have the conventional meanings. In these cases, the bromine atom is chemically very active and participates in the subsequent vulcanization process. The quantities of these various chemical structures may be readily determined by 500 MHz HNMR with high levels of accuracy. The exo allylic bromide structure is the predominant one and usually forms about 75 to about 85 percent of the desired chemical structures. The total primary structure thus refers to the total of the exo allylic bromide, the endo allylic bromide and the re-arranged exo allylic bromide and the product of the present process contains not less than about 80 percent of the bound bromine atoms in these configurations which are the form to actively participate in the subsequent vulcanization process. The process of the present invention leads to a high conversion of the bound 1,4-isoprene units into the primary structure. The present process leads to such conversions of not less than 70 and up to about 90 or more percent into the primary structure whereas the prior art processes lead to such conversions of only about 50 to about 60 percent. The total primary structure as a percentage of the bound 1,4-isoprene units clearly shows the above effect.
The halogenation process may be operated at a temperature of from about 10xc2x0 C. to about 60xc2x0 C., preferably from about 20xc2x0 C. to about 50xc2x0 C. and the reaction time may be from about 1 to about 10 minutes, preferably from about 1 to about 5 minutes. The pressure in the halogenation reactor may be from about 0.8 to about 10 bar.
The brominated polymer recovered from the halogenation process typically has a molecular weight, expressed as the Mooney viscosity (ML 1+8 at 125xc2x0 C.), of from about 25 to about 55. Brominated polymer such as bromobutyl polymer contains from about 0.5 to about 3, most preferably from about 1 to about 2, mole percent of isoprene and from about 97 to about 99.5, most preferably from about 98 to about 99, mole percent of isobutylene based on the hydrocarbon content of the polymer, and from about 1 to about 4, preferably from about 1.5 to about 3, weight percent of bromine based on the bromobutyl polymer. Further, the product of the present process contains not less than about 80 percent of the bound halogen atoms in a form suitable to participate in the vulcanization of the polymer. In the product of the prior art, a small amount of the bound halogen is present in the polymer as a result of the addition of a hydrogen bromide molecule across a carbon-carbon double bond thereby forming a saturated group in which the bromine atom is essentially inert in the vulcanization process. In the product of the present process, the amount of bound bromine present in the polymer as a result of the addition of a hydrogen bromide molecule across a carbon-carbon double bond is reduced from that of the prior art. Analysis of the polymer by 500 MHz HNMR was the method used to determine the structural composition of the brominated polymers. In isobutylene-isoprene polymers, the isoprene bound in the 1,4-configuration in the HNMR spectrum exhibits a resonance at about 5.1 ppm. In the brominated isobutylene-isoprene polymers, the HNMR spectrum exhibits resonances at about 5.4, 5.05 and 4.35 ppm which correspond to the exo allylic bromide structure, at about 5.6 ppm which correspond to the endo allylic bromide structure, at about 4.08 and 4.10 ppm which correspond to the re-arranged exo allylic bromide structure. The hydrobrominated structure is calculated from the mass balance. Also in the product of the present process, the proportion of the carbon-carbon double bonds that have been reacted with a bromine atom (to form an unsaturated bromine containing group which is active in the vulcanization process) is much higher than is the case for the product of the prior art processes. Hence the product of the present process contains a higher proportion of the bound bromine atoms in a form suitable to participate in the vulcanization process.
The brominated polymer is used to make vulcanizates. The vulcanization of polymers is well known. Carbon black is well known in the art for the reinforcement of vulcanizates and is added to the polymer during the compounding process. Hydrocarbon extender oils are also well known materials used in the compounding process. Generally the carbon black will be selected from the furnace and channel carbon blacks and may be used in amounts of from about 20 to about 90 parts by weight per 100 parts by weight of polymer. Hydrocarbon extender oils may be selected from the paraffinic, naphthenic and aromatic oils, preferably from the paraffinic and naphthenic oils, and may be used in amounts from about 5 to about 40 parts by weight per 100 parts by weight of polymer. The vulcanization systems for use with the present product are those already known in the art for use with bromobutyl polymers and generally will include a metal oxide, at least one sulphur based accelerator and, optionally, elemental sulphur. A suitable metal oxide is zinc oxide used in an amount of from about 1 to about 7 parts by weight per 100 parts by weight of polymer. Suitable sulphur based accelerators may be selected from the thiuram sulphides, the thiocarbamates, the thiazyl compounds and the benzothiazyl compounds. The amounts of such accelerators may be from about 0.3 to about 3 parts by weight per 100 parts by weight of polymer. Elemental sulphur may be present in an amount up to about 2 parts by weight per 100 parts by weight of polymer. Various stabilizers, antioxidants, tackifiers, etc. may also be added during the compounding process. The compounding itself will be by conventional methods using a rubber mill or an internal mixer, controlling the temperature to keep it below about 80xc2x0 C., in one or two stages, generally with the cure active components being added last. The so-formed compounds are then shaped and vulcanized by heating for from about 5 to about 60 minutes at temperatures of from about 150xc2x0 C. to about 200xc2x0 C.